The invention relates to downhole rotary pumping arrangements and more particularly to drivehead gimbals and seals used in the driveheads of downhole rotary pumps. The gimbal and seal system described enables the wobble of a rotating shaft to be absorbed without compromising the seal around the shaft.
Downhole rotary pumps, such as progressing cavity pumps, are used for the conveying of different types of fluids, but are especially well suited for the pumping of very viscous or thick liquids such as crude oil laden with sand. A downhole rotary pump is operated by a drive string, generally consisting of a rod or tube string that extends down the well bore. The drive string rotates in a stationary production tubing and is suspended from and rotated by a drivehead assembly which is associated with the wellhead. The drivehead assembly is generally mounted to the top of a wellhead which is attached to the top end of the production tubing. A polished rod is attached to the upper end of the drive string and extends through the drivehead to facilitate sealing around the drive string and backwashing of the pump. The pump includes a stator connected to the bottom end of the production tubing and a rotor attached to the bottom end of the drive string. Upon actuation of the pump by rotation of the drive string, the pumped liquid is forced to the ground surface through the annular space provided between the drive string and the production tubing.
In conventional downhole rotary pump driveheads, a stuffing box is used to seal the annular space between the tubing and the drive string in the drivehead and above the pumped liquid take-off valve of the wellhead. The stuffing box is generally mounted in a bottom end of the drivehead to tightly seal around the rotating polished rod. In order to reduce wear of the polished rod and stuffing box components and to provide adequate lubrication and cooling of the polished rod/stuffing box interface, the fit of the stuffing box around the string must be adjusted such that a controlled leakage of about 2 to 3 drops of pumped liquid per minute is achieved. This constant leakage, although at a relatively low rate, can result in the accumulation of significant amounts of spilled liquid over long periods of operation. Recently, political pressure by environmental groups and the enactment of laws in many jurisdictions which require xe2x80x9ccleanxe2x80x9d wellheads, have forced well operators to reduce the stuffing box leakage rate. However, the resulting lesser lubrication causes considerably higher stuffing box and polished rod wear. Furthermore, wobble of the polished rod, which is especially prevalent with Moineau type pumps must be compensated by the stuffing box if a reliable seal is to be achieved, places additional stress on the sealing material respectively used. Thus, a reliable and durable seal is desired, which will not allow unacceptable leakage as described in Canadian Patent 2,162,311 issued Dec. 22, 1998 and U.S. Pat. No. 5,639,227 and incorporated herein by reference.
However, as a further improvement on the seal, and in order to further compensate for the wobble of a rotating shaft, there has been a need for seal system incorporating a gimbal and seal system.
In accordance with the invention, a seal and gimbal system for use in a drivehead of a downhole rotary pump is provided, which pump is operated by a drive string suspended from the drivehead for rotation in a production tubing about a longitudinal axis, the drivehead having a bore in fluid communication and coaxial with the production tubing and sized to permit the passage of a polished rod attached to a top end of the drive string, the seal and gimbal system for sealing an annular space between the production tubing and polished rod from the drivehead, the seal and gimbal system comprising:
a top seal assembly fixed to the polished rod and rotatable with the polished rod, the top seal assembly also for providing a seal between the annular space and drivehead;
a gimbal assembly operatively connected to the top seal assembly and rotatable with the top seal assembly, the gimbal assembly having first and second portions enabling pivotal displacement of the first and second portions with respect to one another during rotation of the polished rod and a gimbal seal between the first and second portions;
a bearing assembly operatively connected to the gimbal assembly enabling rotation of the gimbal assembly and the polished rod with respect to the drivehead;
a bottom seal assembly operatively connected to the bearing assembly for providing a seal between the drivehead and annular space.
In further embodiments, the first portion includes an inner spherical member having a convex and spherical outer surface and the second portion includes a base member having a concave semi-spherical inner surface engageable with a retaining ring having a concave semi-spherical inner surface, the gimbal seal includes at least one o-ring seated between the first and second portions, and/or the bottom seal assembly includes an inner sleeve radial to the polished rod and at least one rotating seal selected from any one or a combination of a pressure seal, mechanical seal, debris exclusion seal and labyrinth seal between the rotating inner sleeve and drivehead.
In a further embodiment, the system also includes a bushing within the drivehead for stabilizing the polished rod.
In a more specific embodiment, the seal and gimbal system comprises:
a top seal assembly fixed to the polished rod and rotatable with the polished rod, the top seal assembly also for providing a seal between the annular space and drivehead, the top seal assembly including a seal mounting ring and top ring for retaining a seal;
a gimbal assembly operatively connected to the top seal assembly and rotatable with the top seal assembly, the gimbal assembly having first and second portions enabling pivotal displacement of the first and second portions with respect to one another during rotation of the polished rod and a gimbal seal between the first and second portions wherein the first portion includes an inner spherical member having a convex and spherical outer surface and the second portion includes a base member having a concave semi-spherical inner surface engageable with a retaining ring having a concave semi-spherical inner surface and the gimbal seal includes at least one o-ring seated between the first and second portions;
a bearing assembly operatively connected to the gimbal assembly enabling rotation of the polished rod with respect to the drivehead;
a bottom seal assembly operatively connected to the bearing assembly for providing a seal between the drivehead and annular space wherein the bottom seal assembly includes an inner sleeve radial to the polished rod and at least one seal selected from any one or a combination of a pressure seal, mechanical seal debris exclusion seal and labyrinth seal between the inner sleeve and drivehead.